This invention relates to a method for preparing powders of nickel alloy, molybdenum, and chromium carbide/nichrome which involves milling and agglomerating, most typically followed by sintering and plasma processing. The resulting powder when used in thermal spray coating applications produced coatings which are much more uniform and have higher hardness, lower wear rates and friction coefficients when compared to coatings made from blends prepared by prior methods.
Blended powders of molybdenum and nickel self-fluxing alloys are commonly used to produce thermal or plasma sprayed coatings for various applications including piston rings for internal combustion engines. More recently there has been interest to blend chromium carbide to these powders for specific applications. Typically these blends consist of spray dried or densified molybdenum and atomized nickel alloys. When plasma sprayed to produce coatings, the coating microstructure shows large islands of molybdenum and nickel alloy. The size of these islands is controlled by the starting size of the individual component, namely Mo and the Ni alloy. This macrosegregation has its advantages and disadvantages. For instance large unreacted Mo islands are desirable because they provide the low friction coefficient (due to oxide film formation) which is advantageous for piston ring applications. The large Ni alloy rich regions provide wear resistance. However, in coatings made from such powders, while the wear rate is good, once the wear process is initiated, the propagation takes place quite rapidly because the pull-out regions are large.
Therefore it would be desirable to improve overall wear characteristics of thermal spray coatings. The hardness and wear characteristics can be further improved by addition of certain hard phases to the existing compositions of Mo/Ni alloys.